Shorten TARGET_BRAND from generic_<arch> to Android

Switching to userdebug emulator images increased ro.build.fingerprint
Past the 91 character limit.  This should bring the build server
builds under this limit.

Fixing it for local builds will be a bit more difficult.

Bug: 25829506

Change-Id: I7daee3478716adcb860cced1b770a2d00a72f1a2
(cherry picked from commit efbeab78d3581009b3da20991a3a83d733393f45)